The present invention relates to a new and improved construction of apparatus for the application of a protective coating to the welded or soldered seam of can bodies.
Generally speaking, the apparatus of the present invention is of the type comprising a first device for feeding or conveying the can bodies along a path of travel and a second device arranged along the center line of such path of travel. The second device comprises at least one roll or roller which is partially immersible in a coating mass for applying such coating mass to the inner surface or inside of the welded seam.
Present day technology concerned with surface protection of cans against corrosion effects applies a protective layer, for instance a lacquer layer, by rolling such on to the uncut plates. After the cutting of the blanks from such lacquer-coated plates there are formed the can bodies. To achieve a faultless welding of the seam the part of the sheet metal of the can body at the region of the seam is devoid of any protective layer. Following the welding operation these bare portions of the can bodies likewise must be covered with a lacquer coating.
With conventional techniques this is accomplished by spraying the protective lacquer by means of a spray gun. With this method, during the spraying operation, additional air is entrapped beneath the lacquer coating so that during the drying operation there can thus form pores in the protective coating. Optimum covering of the welded seam is particularly then of paramount importance when filling the cans with foodstuffs, to thus prevent any danger of food poisoning.
According to another operating procedure the lacquer is applied to the welded seam by means of an applicator roll driven by the cans. Here, however, there is needed a large contact pressure between the cans and the applicator roll, in order to sufficiently place into rotation such applicator roll. This, in turn, can have adverse effects upon the lacquer coating of the welded seam. If no cans are fed then the lacquer applicator roll is stationary and the lacquer dries-up. This effect equally is unfavorable for the subsequent application of the protective coating. A further drawback which has been found when practicing this method is that the first number of cans which are processed are only partially provided with a lacquer coating and therefore must be rejected. This, in turn, requires sorting operations which obviously increase production costs.